Oxidation of methylaromatic hydrocarbons in the presence of a promoter



United States Patent 3,256,324 OXXDATION 0F METHYLAROMATIC HYDROCAR- BONS IN THE PRESENCE OF A PROMOTER Phillip S. Landis, Woodbury, David D. Neiswender, Lawrence Township, Mercer County, and Robert D. Offenhauer, Sewell, N..J., assignors to Socony Mobil Oil Company, Inc., a corporation of New York No Drawing. Filed Aug. 26, 1963, Ser. No. 304,614 11 Claims. (Cl. 260524) This application is a continuation-in-part of our. application for Uni-ted States Letters Patent, Serial No. 19,- 500, filed April 4, 1960, now abandoned.

This invention relates to the manufacture of aromatic carboxylic acids. It is more particularly concerned with a process for oxidizing a methyl-substituted aromatic compound to the corresponding carboxylic acid or acids.

As is well known to those skilled in the art, it has been proposed to oxidize aromatic compounds, particularly alkyl-substituted aromatic hydrocarbons, to produce aromatic carboxylic acids. The alkylaromatic hydrocarbons having two or more carbon atoms in the alkyl group have been oxidized with relative ease. The oxidation of methyl-substituted aromatic hydrocarbons, on the other hand, has been relatively difficult. Even with catalysts, temperatures of the order of 150-225 C. and higher have been required, for example, to oxidize xylene to toluic acid. In order to maintain a liquid phase, superatmospheric pressures were necessary.

It has now been found that methyl-substituted aromatic compounds (methylaromatics) can be oxidized in the liquid phase at atmospheric pressure. It has been discovered that certain branched-chain alkyl-substituted aromatic hydrocarbons act as promoters for the oxidation of methylarom-atic compounds to aromatic carboxylic acids at atmospheric pressure. Accordingly, it is a broad object of this invention to provide a process for producing aromatic carboxylic acids. Another object is to provide a method for oxidizing methylaromatic compounds to carboxylic acids. A specific object is to provide a process for oxidizing methylaromatic compounds to aromatic carboxylic acids at atmospheric pressure. Another specific object is to provide a process for oxidizing methylaromatic compounds to aromatic carboxylic acids that is promoted by certain bran-ched chain alkyl-substituted aromatic hydrocarbons.

In general, the present invention provides a method for producing aromatic carboxylic acids that comprises establishing a reaction mixture of a methylaromatic compound and an alkylaromatic hydrocarbon promoter, wherein the alkyl group has a secondary carbon atom directly attached to the aromatic nucleus, and the Weight ratio of said methylaromatic compound to said alkylaromatic promoter being between about 0.121 and about :1, maintaining said reaction mixture at a temperature varying between about 50 C. and the boiling point of said methylaromatic compound; contacting said reaction mixture with oxygen-containing gas, with or Without an oxidation catalyst, at a how rate varying between about 0.1 cubic foot per hour and about 4 cubic feet per hour, for a period of time varying between about 0.5 hour and about 10 hours; and recovering from the reaction mixture an aromatic carboxylic acid.

Theprocess of this invention is applicable to the oxidation of methyl-substituted aromatic hydrocarbons to. the

corresponding aromatic carboxylic acids at temperatures sufficiently low to permit liquid phase operation at atmospheric pressure. Examples of the type of oxidation contemplated herein include the oxidation of toluene to hen-- zoic acid, the oxidation of xylene to toluic acid, and the oxidation of methylnaphthalene to naphthoic acid. It

"ice

has also been found that methyl-substituted aromatic carboxylic acids can be directly oxidized to polycarboxylic aromatic acids, without resort to converting the methylaromatic carboxylic acid to the ester. Thus, for example, it is contemplated to oxidize p-toluic acid to terephthalic acid, without first converting to the methyl toluate ester.

Accordingly, non-limiting examples of the methyl-substituted aromatic compounds oxidizable in the process of this invention include, toluene, o-xylene, m-xy-lene, pxylene, hemimellitene, psuedocumene, mesitylene, a-rnethylnaphthalene, ,6 methylnaphthalene, dimethylnaphthalene, o-toluic acid, m-toluic acid, p-toluic acid, hemimellitic acid, xylic acid, mesitylenic acid, methylpyridine, pch lorotoluene, tolunitrile.

It is a discovery of this invention that the aforedescribed methylaromatic compounds, which are diflicult to oxidize at atmospheric pressure, are readily oxidized when a promoter is used. The promoters contemplated herein are alkylaromatic hydrocarbons in which the alkyl group has a secondary carbon atom directly attached to the aromatic nucleus. These compounds have a structure:

wherein R and Rf are alkyl groups containing 1-3 carbon atoms. These compounds have one hydrogen atom in a position alpha to the aromatic nucleus. Non-limiting examples of promoters include cumene, isobutylbenzene, isoamylbenzene, 3-phenylpentane, 3-phenylhexane, and 4- phenylheptane. Cumene is particularly preferred from the standpoint of its ready availability and effectiveness.

The amount of promoter used, in proportion to the amount of methylaromatic charge, can vary somewhat. Generally, the weight ratio of methylaromatic compound to promoter will vary between about 10:1 and about 0.1 1. Preferably, it will be between about 4:1 and about 0.2:1.

The oxidation reaction is carried out in the liquid phase by contacting the methylaromatic-promoter mixture with oxygen in the presence of a catalyst, at temperatures which permit operation at atmospheric pressure. In the absence of the promoter, the oxidation reaction (as previously had been proposed) requires elevated temperatures which are considerably higher than the boiling point of the methylaromatic compound. Accordingly, superatmospheric pressures have been required to maintain liquid phase. The present process operates at temperatures ranging from about 50 C. up to the boiling point of the methylaromatic compound reactant. For example, toluene (B.P.=ll0.8 C.) can be oxidized readily, by the process of this invention, at about 110 C. Xylenes (B.P.=138-140 C.) are processed readily at aboutv 135 C. In general practice under the present invention,

. the highest reaction temperature will be within about 15 C. of the boiling point of the methylaromatic compound reactant.

Oxygen is supplied to the process as relatively pure oxygen or in the form of a molecular oxygen-containing gas, such as air. The amount of oxygen used (measured in terms of molecular oxygen, in the case of oxygencontaining gas) will vary between about 0.1 cubic foot hour and about 5 hours.

The use of the aforedescribed alkylaromatic hydrocarbon promoter, such as cumene, to promote-the oxidation of methylaromatic hydrocarbons has a general application. Thus, the promotion effect has been noted even in the absence of an oxidation catalyst. The effect, however, is less pronounced, i.e. relatively lower conversions to carboxylic acids are noted for the same reaction conditions. In general, therefore, the alkylaromatic hydrocarbon promoter will effect improved yields in any liquid phase, catalytic process for oxidizing methylaromatic hydrocarbons to aromatic carboxylic acids. In its broadest aspect, the promotional eifect of this invention is not dependent upon the particular oxidation catalyst selected and will be observed with any of the oxidation catalysts known to the art.

Various types of oxidation catalysts used in the art have been extensively investigated and found elfective in this invention. One preferred type is a heavy metal phthalocyanine or porphorazine, such as, for example, iron phthalocyanine. Another type of catalyst includes metal salts of organic carboxylic acids, such as, cobalt naphthenate, cobalt permanganate, cerium chloride, copper phosphate, vanadium chloride, and manganese acetate. The more eifective catalysts of this type are those wherein the metal is in its lower valence state, e.g. cobaltous naphthenate or acetate. Generally speaking, the oxidation catalysts commonly used include the solid polyvalent metals having an atomic weight between about 50 and 200. The metals, themselves, may be used in the finely divided metallic state, or as oxides or as compounds of the types already referred to. Mixtures of two or more oxidation catalysts may be utilized if desirable.

In the process of this invention, part of the alkylaromatic promoter ,will be oxidized. In the case of cumene, some will be converted to acetophenone, a material useful as a perfume ingredient and as lacquer thinner. The main products, aromatic carboxylic acids, can be removed in any of the various ways for separating organic acids from hydrocarbons. Thus, the carboxylic acid can be neutralized by adding an aqueous solution of alkali, such as aqueous sodium or potassium hydroxide, to the reaction product to obtain an aqueous solution of the salt of the aromatic carboxylic acid. The'aqueous solution is separated from the organic material (unreacted reactant, ketones, etc.) and then acidified to release the aromatic 4 amples, or by the operations or manipulations involved. .As will be apparent to those skilled in the art, other reactants and conditions, as set forth herein, can be used to practice this invention.

OXIDATION OF TOLUENE vExample 1 A mixture of 80 grams toluene, 20 grams cumene (promoter), and 0.5 gram cobalt naphthenate (catalyst- 4.5% Co) was heated at 110 C. in a reaction vessel equipped with a reflux condenser attached to a small water take-off device to azeotropically remove water from the reactants as it was formed. Oxygen was bubbled through the heated reaction mixture at the rate of 0.5 cubic foot per hour, for three hours. The reaction mixture was cooled and extracted with ml. of 10% aqueous solution of sodium hydroxide. The caustic extract was neutralized with 10% hydrochloric acid and the solid acid that separated was removed by filtration. The crude acid (31 grams) was recrystallized from hot Water to yield a product melting at 119121 C. (Reported for benzoic acid=12l C.). The acid number was 459 (theory for benzoic acid=460). The caustic-insoluble oil fraction was analyzed by vapor phase chromatographic analysis and showed 8% acetophenone, i.e. conversion of about 40% of the cumene to acetophenone. Unreacted toluene and cumene are recoverable by distillation for recycle to the process.

Example 2 Another run similar to that of Example 1 was made, except that no cumene promoter was used. After 3% hours operation at 110 C. using an oxygen flow rate of 0.5-0.6 cubic foot per hour, there was produced no benzoic acid.

Examples 3 and 4 Two runs were made, as described in Example 1, using air as the source of oxygen. The flow rates were 0.7-0.8 cubic foot of air per hour, equivalent to an oxygen flow rate of about 0.14-0.16 cubic foot per hour. In one run the ratio of toluene to cumene was 4:1 and in the other, 121. Pertinent data on these run and on the runs of Examples 1 and 2 are summarized in Table I.

TABLE I Examples Wt. percent Conv. Toluene to Bcnzoic Acid Wt. percent Conv. Curnene to Acetophenone Time, Temp.,

Hrs. O.

Reactants Oumene, 20 g Toluene,

0 0.5 cu.ft./l Toluene, 100 g Cobalt Naphthenate, 0.5 g. Oz 0.5-0.6 cu.ft./hr Oumene, 20 g Toluene, 80 g Cobalt Naphthenate, 0 Air O.70.8 cu.ft./hr. Cumene, 50 g Toluen Cobalt Air 0.8 cu.ft./hr

Naphthenate, 0.5 g.

OXIDATION OF XYLENE Example 5 In a reaction vessel provided with reflux condenser and water take-off trap, were placed 50 grams p-xylene, 50 grams cumene (promoter), and 0.5 gram iron phthalocyanine (catalyst). This reaction mixture was maintained at about C. by externally heating the reaction vessel with refluxing xylene. Oxygen was bubbled through the heated reaction mixture at a rate of 0.3-0.4 cubic foot per hour for 2 hours. The reaction mixture was then cooled in an ice bath, whereupon crystals separated. These were removed by filtration, yielding a crude product weighing 17 grams. This product was recrystallized from an alcohol-water mixture to yield clean white crystals melting at 176-9 C. (p-toluic acid melts at poured carefully into cold, concentrated ammonium hydroxide and the precipitated amide was separated and recrystallized from water. The dried amide melted at 1456 C. Reported melting point of orthotoluarnide 179 C.). The white crystalline product when mixed 5 is 147 C. I with known p-toluic acid (mixed melting point) did The oil remaining after caustic extraction welghed not depress the melting point (179 C.) of the known 84 grams. It was examined by vapor phase chromatogp-toluic acid. raphy and found to contain 76.1% cumene and o-xylene, The white crystalline product was further identified 19.6% acetophenone, and small amounts of dimeric as p-toluic acid by treating 0.2 gram with ml. thionyl products. The run is summarized in Table II. chloride under reflux forminutes and then pouring the mixture into cold concentrated ammonium hydrox- Example 8 ide. The precipitated amide was dried and found to melt at 160-3 C. (p-toluamide melts at 165 C.). run Sunllar to that Egg? 7 was gg except The oil recovered after filtration of the crude acid 15 iy i l g i gigg i p i z i g g fg g i gg product was analyzed by vapor phase chromatography o and found to contain 50.8% cumene and p-xylene and i g h i at for 3 hours at a 25.9% acetophenone. The remaining material cona F per E i was measur' tained dicumyl and small amounts of higher molecular g i anon pro 6 run 18 Summarweight products. The cumene and p-xylene unreacted 12c m a e E l 9 can be recycled to the process. The run is summarized xamp e in Table 11. Using the reaction system and technique of the run of Example 7, a charge of 50 grams m-xylene, 50 grams Example 6 cumene, and 0.5 gram iron phthalocyanine was oxidized for 3.2 hours at 135 C., using an oxygen rate of 0.3-0.4 A fun slmllar to that Of Example 5 Was made except cubic foot per hour. A crude acid product was obtained t at 110 6111118116 Was used: The Charge W 75 grams (14 grams) which, when recrystallized from water, pand 0375 gram lfOIl phthalocyanlh? Oxygen melted at l079 C. Reported melting point of metawas bubbled through the charge at 135 C. for 5 hours, toluic acid is 110 C a a rate cuhl? Q P hour- There Y no The oil recovered after caustic extraction weighed 84 measurable yield of oxidation product. The run is sumgrams Vapor phase chromatography establish; it as marized in Table IL 70.0% m-xylene and cumene, 18.9% acetophenone, and

l minor amounts of dimeric products. The run is sum- Examp e 7 3 marized in Table II.

0 Using the reaction system described in Example 5, a Example 10 arge of hgh 811115 fY 7 g t ii fi gg g A run was made as described in Example 9, except that gram Iron P t a Ocyanme was e at no cumene promoter was used. The charge of 100 grams g6I1 WE1S bubbled through the Charge at rate of m-xylene and 0.5 gram iron phthalocyanine was oxidized Cublc fOOt P 110111 for 3 hours- The Product was 40 for 24 hours at 135 C., using an oxygen rate of 0.3 cubic cooled and extracted with 200 ml. of 10% potassium foot per hour. No measurable yield of oxidation product hydroxide. The aqueous caustic extract was separated wa obtained, Th run i Summarized i T bj 1L TABLE II Reaction Conditions Percent Solid Oonv. Percent Examples Reactants Acids, Wt. Xylene to Aceto- Oz Input, Temp., Time, Percent Acid phenone cu. it./hr. 0. Hrs.

5 {;-Xyle11e,5g0 g 0 Fullg ei na egg 13 2 17 34 26 6 {%1 i%?0l7 5"gt:::} 135 5 Nil Nil 7 d ia GiJ g 0 34) 4 135 umen Fe PTO, 0.6 g 3 12 24 20 8 g: 0.3 135 3 Nil Nil 9 d o g l 0 3-0 4 1 Fe PTO, e

$Z o. 3 135 24 Nil Nil 1 Fe PTO is iron phthalocyanine.

From the data in Table II it will be appreciated that good yields of toluic acids are obtained by the process of this invention, using liquid phase operation at atmospheric pressure. The acetophenone produced is a utilizable byproduct. Unreacted xylene and cumene can be recycled to extinction.

OXIDATION OF OTHER METHYLAROMATIC COMPOUNDS Example 11 Mesitylene (60 grams), cumene (60 grams) and iron phthalocyanine (0.6 gram) were heated at C. for 5 A2 hours whileoxygen was bubbled through the solution.

The cooled oxidation product was extracted with three 100 ml. portions of 10% sodium hydroxide solution, leaving 90 grams of caustic-insoluble oil. Neutralization of the caustic extract with 10% hydrochloric acid produced grams of crude acid, which was crystallized from water to yield while crystals having a melting range of 157160 C. The reported melting point of mesitylenic acid is 166 C. The acid number of the acid was 368 (theoryfor mesitylenic acid, 374).

Example 12 A solution containing 20 grams of p-toluic acid, 80 grams of cumene and 0.5 gram of iron phthalocyanine were heated at 135 C. while oxygen was passed through the solution at a rate of 0.4 cubic foot per hour for 3 hours. The solution was filtered hot and 2.35 grams of solid acid were separated in this fashion (terephthalic acid is extremely insoluble in most organic solvents while p-toluic acid is soluble in hot organic solvents). The solid wasdigested with two 400 ml. portions of hot water and the insoluble solid separated by filtration and dried in a drying pistol, weight 2.32 grams. This solid did not melt when heated to 220 C. and gave an acid number with two breaks (indicative of two carboxyl groups). Acid numbers found:' 337, 453; theoretical for terephthalic acid: 338, 674. (Solubility problems render it ditficult to obtain a satisfactory second break in the acid number determination for pure terephthalic acid.)

Example- 13 Sixty grams of p-xylene and 60 grams of cumene were oxidized (note: no oxidation catalyst was used) at 135 C. for 7 hours using oxygen at a flow rate of 0.3 cubic foot per hour. The crude p-toluic acid obtained weighed 6 grams and was similar to that prepared in Example 5. 1

Recrystallization gave a solidp-toluic acid of melting point 1748 C. which did not depress the melting point of known p-toluic acid when added thereto.

Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be resorted to, Without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such variations and modifications are considered to be within the purview and scope of the appended claims.

What is claimed is:

1. A method for producing aromatic carboxylic acids that consists essentially of establishing a reaction mixture of a rnethylaromatic hydrocarbon and an alkylaromatic hydrocarbon promoter having the structure:

wherein R and R are alkyl groups having 1-3 atoms, and the weight ratio of said methylaromatic hydrocarbon and said alkylaromatic promoter being between about 0.1:1 and about 10:1; maintaining said reaction mixture at a temperature varying between about 50 C. and about 135 C. and under atmospheric pressure; and contacting said reaction mixture in the liquid phase with a molecular oxygen-containing gas at a flow rate, measured in terms of oxygen, varying between about 0.1 cubic foot per hour and about 4 cubic feet per hour per 100 g. of said reaction mixture, and for a period of time varying between about 0.5 hour and about 10 hours.

2. The process defined in claim 1, wherein said process is carried out in the presence of a heavy metal oxidation catalyst.

3. A method for producing benzoic acid that consists essentially of establishing a reaction mixture of toluene wherein R and R are alkyl groups having 1-3 C atoms, and the weight ratio of said toluene to said alkylaromatic promoter being between about 0.2:1 and about 4: 1; maintaining said reaction mixture at a temperature varying between about 50 C. and about 110 C.; and under atmospheric pressure and contacting said reaction mixture with a molecular oxygen-containing gas at a flow rate, measured in terms of oxygen, varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per g. of said reaction mixture, in the presence of cobalt naphthenate catalyst, and for a period of time varying between about one hour and about 5 hours.

4. A method for producing benzoic acid that consists essentially of establishing a reaction mixture of toluene and cumene promoter, the weight ratio of said toluene to said cumene being between about 0.221 and about 4: l; maintaining said reaction mixture at a temperature of about C. and under atmospheric pressure; contacting said reaction mixture with oxygen at a fiow rate'varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture, in the presence of cobalt naphthenate catalyst, and for a period of time varying between about one hour and about 5 hours; and recovering benzoic acid from the reaction mixture.

5. A method for producing benzoic acid that consists essentially of establishing a reaction mixture of toluene and cumene promoter, the weight ratio of said toluene to said cumene being between about 0.221 and about 4: 1; maintaining said reaction mixture at a temperature of about 110 C. and under atmospheric pressure; contacting said reaction mixture with air at a flow rate, measured in terms of oxygen, varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture, in the presence of cobalt naphthenate catalyst, and for a period of time varying between about one hour and about 5 hours; and recovering benzoic acid from the reaction mixture.

6. A method for producing toluic acids that consists essentially of establishing a reaction mixture of xylene and cumene promoter, the weight ratio of said xylene to said cumene being between about 0.211 and about 4:1; maintaining said reaction mixture at a temperature varying between about 50 C. and about C. and under atmospheric pressure; and contacting said reaction mixture with oxygen at a flow rate varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture, and for a period of time varying between about 0.5 hour and about 5 hours.

7. A method for producing paratoluic acid that consists essentially of establishing a reaction mixture of pxylene and cumene promoter, the weight ratio of said p-xylene to said cumene being between about 0.2:1 and about 4: 1; maintaining said reaction mixture at a temperature of about 135 C. and under atmospheric pressure; contacting said reaction mixture with oxygen at a flow rate varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture in the presence of iron phthalocyanine catalyst, and for a period of time varying between about one hour and about 5 hours; and recovering paratoluic acid from the reaction mixture.

8. A method for producing orthotoluic acid that consists essentially of establishing a reaction mixture of oxylene and cumene promoter, the Weight ratio of said 0- xylene to said cumene being between about 0.2:1 and about 4:1; maintaining said reaction mixture at a temperature of about 135 C. and under atmospheric pressure; contacting said reaction mixture with oxygen at a flow rate varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture, in the presence of iron phthalocyanine catalyst, and for a period of time varying etween about one hour and about 5 hours; and recovering orthotoluic acid from the reaction mixture.

9. A method for producing metatoluic acid that consists essentially of establishing a reaction mixture of mxylene and cumene promoter the weight ratio of said mxylene to said cumene being between about 02:1 and about 4:1; maintaining said reaction mixture at a temperature of about 135 C. and under atmospheric pressure; contacting said reaction mixture with oxygen at a flow rate varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture, in the presence of iron phthalocyanine catalyst, and for a period of time varying between about one hour and about 5 hours; and recovering metatoluic acid from the reaction mixture.

16'. A method for producing mesitylenicacid that consists essentially of establishing a reaction mixture of rnesitylene and cumene promoter, the weight ratio of said mesitylene to said cumene being between about 0.211 and about 4:1; maintaining said reaction mixture at a temperature of about 135 C. and under atmospheric pressure; contacting said reaction mixture with oxygen at a flow rate varying between about 0.2 cubic foot per hour and about 0.5 cubic foot per hour per 100 g. of said reaction mixture, in the presence of iron phthalocyanine catalyst, and for a period of time varying between about one hour and about 5 hours; and recovering mesitylenic acid from the reaction mixture.

11. A method for producing terephthalic acid that consists essentially of establishing a reaction mixture of p- References Cited by the Examiner UNITED STATES PATENTS 2,245,528 6/1941 Loder 260-524 2,833,816 5/1958 Safier et a1 260-524 3,139,452 6/ 1964 Hay 260524 FOREIGN PATENTS 681,455 10/1952 Great Britain.

787,054 11/1957 Great Britain.

801,387 9/1958 Great Britain.

LORRAINE A. WEINBERGER, Primary Examiner.

M. S. JAROSZ, L. A. THAXTON, Assistant Examiners. 

1. A METHOD FOR PRODUCING AROMATIC CARBOXYLIC ACIDS THAT CONSISTS ESSENTIALLY OF ESTABLISHING A REACTION MIXTURE OF A METHYLAROMATIC HYDROCARBON AND AN ALKYLAROMATIC HYDROCARBON PROMOTER HAVING THE STRUCTURE: 